This invention relates to an apparatus for monitoring a device powered by the apparatus, and a method for monitoring a device from its power draw. The invention is particularly useful in the field of irrigation systems, however the invention may also be applied to other fields.
In many areas of the world the availability of water to maintain the natural growth of plants is either insufficient or unreliable, especially if the plants are not native to the area. For centuries this problem has been overcome by the development of irrigation systems where water is transferred from a local available source such as a river, dam or bore and used to artificially irrigate the plants.
The twentieth century has seen the further development of irrigation systems to the level of total computerised automation. This has resulted in many areas of the world having large tracts of previously unusable, arid land that are now capable of producing crops of all types.
A typical irrigation system comprises a network of underground pipes along which water is pumped. Selected valves at strategic points on this network, when activated, release water to local distribution points such as sprinklers or drippers. The method of activating these valves may vary, but typically they would be triggered by electrical, mechanical, hydraulic or manual means.
The most common electrical device is an electro-mechanical solenoid. An activating current causes the solenoid to move a spring-loaded plunger, allowing the valve to open due to the water pressure in the irrigation pipes. When this current is either removed or possibly reversed, the plunger returns to its original state thus allowing the valve to close.
The solenoids are activated, either directly or remotely, by an electrical or electronic control system such as irrigation controllers, programmable logic controllers (PLC""s) or even manual switches.
The most common form of irrigation solenoid is activated on application of a voltage of 24 volts AC. Other solenoids activate on a range of different voltages from 6 to 48 volts, either being AC or DC. In order to minimise power consumption, latching solenoids are available which enable on the receipt of a voltage pulse of one polarity and disable when a voltage pulse of the reverse polarity is received.
The typical means of transferring the current required to activate these solenoids are a pair of cables running for distances of up to two kilometers from the controlling system. The limitations on this distance are dependent on the resistance of the cable such that sufficient power is available to activate the solenoid for the required time.
Commercial irrigation sites such as farms, parks or golf courses can cover large areas, consequently the length of cabling required to service all the solenoids may run to many kilometers. Currently there are two main techniques in use to distribute power to the solenoids, referred to as xe2x80x98Direct Connectionxe2x80x99 and xe2x80x98Two-Wirexe2x80x99. A brief description of these techniques follows.
Direct connection is the older or more traditional method, which is to supply power directly from an activating relay (or similar electronic device) within a control system by a directly connected pair of cables. It should be noted that the word xe2x80x98pairxe2x80x99 only refers to the connection point at the solenoid, as the typical wiring layout of such an installation is normally a matrix of single cables with the xe2x80x98pairsxe2x80x99 only occurring at the required solenoid junction locations.
Two-wire systems provide both power and activating commands along a single network. This network generally consists of a true xe2x80x98pairxe2x80x99 of cables and each solenoid within the network is activated by a corresponding decoder connected between it and the network. A master irrigation controller powers and issues commands to the decoders via the pairs of cables. The format of the command communications depends on the manufacturer""s preference. Many existing systems utilise tone or DTMF (Telephone-type tones) signals superimposed on the powering voltage. Normally (and preferably) the network is wired in a xe2x80x98point to pointxe2x80x99 configuration between the master irrigation controller and the decoders.
Most control systems activate solenoids by applying a 24 v AC 50 Hz RMS power signal to the solenoid. Although this technique appears both obvious and simple, a number of problems and limitations do occur.
A typical solenoid used requires around 3 watts at 24 v AC to hold in, resulting in a holding current of around 300 mA. When the solenoid is activated, the inrush current can be double (or more) the holding current. The inrush current must be maintained until the plunger has fully seated.
One example of inrush current increase in duration is where a solenoid plunger is clogged with sediment or sand. On activation, if the force of the solenoid is not sufficient to move the clogged plunger, the plunger would vibrate violently at the waveform frequency and could take a number of seconds to activate. In this case the instantaneous inrush current would have to be maintained for far longer periods before the solenoid would be fully activated. If this solenoid was being activated some distance from the voltage source (the irrigation controller) or if other solenoids were also being activated which used common cabling runs, the resistance of the wire could cause the following scenarios to occur:
The solenoid would not activate.
The voltage drop and solenoid-induced interference at the decoder could be sufficient to cause the decoder electronics to reset, fail, or run unreliably.
If the irrigation controller is equipped with current sensing, it could shut down the section being irrigated and skip to the next section.
The current drawn (under worst cases) could cause a fuse to blow or trip at the irrigation controller. In this case irrigation could be suspended or cancelled.
Throughout the specification, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
In accordance with a first aspect of this invention, there is provided an apparatus for monitoring a device powered by the apparatus, comprising:
Power means arranged to provide an alternating power signal to the device;
Monitoring means arranged to measure a prescribed characteristic of said alternating power signal; and
Control means responsive to said measurements from the monitoring means and arranged to analyse said measurements to determine the status of the device therefrom.
Preferably, the control means is arranged to restart its analysis each one-half cycle of the alternating power signal.
Preferably, said prescribed characteristic is current.
Preferably, the control means is arranged to determine from a previous measurement and the current measurement whether any deviation in the prescribed characteristic exceeds a tolerance value.
Preferably, the control means is arranged to increment a first counter if the prescribed characteristic exceeds the tolerance value.
Preferably, the control means is arranged to increment a second counter if the prescribed characteristic does not exceed the tolerance value.
Preferably, the control means is arranged to calculate an incline value between the current measurement and the previous measurement, and to compare this incline value to a previously calculated incline value, where if the difference between the two incline values is greater than a tolerance value, the control means increments the first counter, otherwise the control means increments the second counter.
Preferably, at the end of each half cycle of the alternating power signal, the control means is arranged to compare the values of the first and second counters and to determine therefrom whether the device is in a first state or a second state.
Preferably, at the end of each half cycle the alternating power signal, the control means is also arranged to compare a maximum current drawn during the half cycle to a threshold value, and if the threshold value is exceeded to determine that the device is in a third state.
In accordance with a second aspect of this invention, there is provided a method for monitoring a device from its power draw, comprising:
providing an alternating power signal to the device;
measuring a prescribed characteristic of said alternating power signal; and
analysing said measurements to determine the status of the device therefrom.
Preferably, the method further comprises the step of restarting the analysis each one-half cycle of the alternating power signal.
Preferably, said prescribed characteristic is current.
Preferably, the step of analysing comprises determining from a previous measurement and the current measurement whether any deviation in the prescribed characteristic exceeds a tolerance value.
Preferably, the step of analysing further comprises the step of counting how often the prescribed characteristic exceeds the tolerance value.
Preferably, the step of analysing further comprises the step of counting how often the prescribed characteristic does not exceed the tolerance value.
Preferably, the step of analysing further comprises the step calculating the incline between the current measurement and the previous measurement, and to compare this incline value to a previously calculated incline value, and counting how often the difference between the two incline values is greater than, and not greater than, a tolerance value.
Preferably, the step of analysing further comprises comparing, at the end of each half cycle of the alternating power signal, the counts of how often the tolerance value is exceeded to determine therefrom whether the device is in a first state or a second state.
Preferably, the step of analysing further comprises comparing, at the end of each half cycle of the alternating power signal, a maximum current drawn during the half cycle to a threshold value, and if the threshold value is exceeded to determine that the device is in a third state.